The Astrophysical Journal, 771:34 (13pp), 2013 July 1 C 2013.
doi:10.1088/0004-637X/771/1/34
The American Astronomical Society. All rights reserved. Printed in the U.S.A.
PETROLOGIC CONSTRAINTS ON AMORPHOUS AND CRYSTALLINE MAGNESIUM SILICATES: DUST FORMATION AND EVOLUTION IN SELECTED HERBIG Ae/Be SYSTEMS 1
Frans J. M. Rietmeijer1 and Joseph A. Nuth2 Department of Earth and Planetary Sciences, MSC 03 2040, 1-University of New Mexico, Albuquerque, NM 87131-001, USA;
[email protected] 2 Astrochemistry Laboratory, Solar System Exploration Division, Code 691, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Received 2012 October 30; accepted 2013 May 4; published 2013 June 12
ABSTRACT The Infrared Space Observatory, Spitzer Space Telescope, and Herschel Space Observatory surveys provided a wealth of data on the Mg-silicate minerals (forsterite, enstatite), silica, and “amorphous silicates with olivine and pyroxene stoichiometry” around Herbig Ae/Be stars. These incredible findings do not resonate with the mainstream Earth Sciences because of (1) disconnecting “astronomical nomenclature” and the long existing mineralogical and petrologic terminology of minerals and amorphous materials, and (2) the fact that Earth scientists (formerly geologists) are bound by the “Principle of Actualism” that was put forward by James Hutton (1726–1797). This principle takes a process-oriented approach to understanding mineral and rock formation and evolution. This paper will (1) review and summarize the results of laboratory-based vapor phase condensation and thermal annealing experiments, (2) present the pathways of magnesiosilica condensates to Mg-silicate mineral (forsterite, enstatite) formation and processing, and (3) present mineralogical and petrologic implications of the properties and compositions of the infrared-observed crystalline and amorphous dust for the state of circumstellar disk evolution. That is, the IR-observation of smectite layer silicates in HD142527 suggests the break-up of asteroid-like parent bodies that had experienced aqueous alteration. We discuss the persistence of amorphous dust around some young stars and an ultrafast amorphous to crystalline dust transition in HD 163296 that leads to forsterite grains with numerous silica inclusions. These dust evolution processes to form forsterite, enstatite ± tridymite could occur due to amorphous magnesiosilica dust precursors with a serpentine- or smectite-dehydroxylate composition. Key words: circumstellar matter – methods: laboratory – methods: observational – protoplanetary disks – stars: variables: T Tauri, Herbig Ae/Be
petrology) to determine the pressure and temperature regimes of mineral and mineral assemblage formations. Earth Scientists have an advantage. They have the sample in hand that they want to simulate in the laboratory, but this kind of ground truth to assess the value and uniqueness of a laboratory simulation experiment does not exist for space dust. What does the name imply for the primary mineral properties (crystallography; chemistry) of forsterite around Herbig Ae/Be stars? Juh´asz et al. (2010) cited three possibilities proposed in the literature, viz. (1) condensation followed by thermal annealing, (2) thermal annealing of pre-existing dust grains heated by shocks, and (3) episodic crystal formation via annealing in the surface layers of protoplanetary disks during accretion outbursts. As all options invoke annealing, it presumes that precursor materials were probably “amorphous silicates with olivine and pyroxene stoichiometry,” which then begs the question of their origin(s). The third option is a wild card. This environment is clearly outside the constraints of the Principle of Actualism but it does not mean that the proposed formation process could not produce crystalline Mg-silicate minerals. The type of shocks in dusty environments implied by the second option has no terrestrial analog but the formation of crystalline forsterite and enstatite by this process could be verified experimentally. The first option invokes condensation and thermal annealing. On Earth, gas-to-solid condensation occurs in hot springs, fumaroles, and active volcanoes. While silica (SiO2 ) formation was reported, there are no reports of the vapor condensation of crystalline forsterite and enstatite, or “amorphous silicates with olivine and pyroxene stoichiometry” that could be precursors for thermal annealing. Eifelite, which is a cordierite-like Na,K-bearing (low-Al) magnesium ring-silicate, could form by metastable vapor phase condensation (Rietmeijer & Nuth 2011).
1. INTRODUCTION “On the basis of the analyzed spectra, the major constituents of protoplanetary dust around Herbig Ae/Be stars are amorphous silicates with olivine and pyroxene stoichiometry, crystalline forsterite, and enstatite and silica” is a quote from the paper “Dust evolution in protoplanetary disks around Herbig Ae/Be stars—The Spitzer view” (Juh´asz et al. 2010). It is cited here for using the terms “crystalline forsterite,” “enstatite,” and “silica,” which are established mineralogical terms. In making these mineral identifications around Herbig Ae/Be stars, Astronomers enter the worlds of Petrology, Mineralogy, and Geology, or in general the Earth Sciences. Each term is the key to an extensive database of crystallographic and chemical properties, occurrences, conditions, and environments of formation. When identifying minerals in extraterrestrial environments, one does well then to recall the founding principle of Geology as it was put forward by James Hutton (1726–1797): “No powers are to be employed that are not natural to the globe, no action to be admitted except those of which we know the principle”. It is known as Actualism. Earth Scientists have to accept that the processes operating in the past on Earth proceeded as we can observe their actions today on Earth. The processes that shaped the Lunar rocks collected by Apollo astronauts, the dust of comet 81P/Wild 2 (Brownlee et al. 2006), and the meteorites can be understood within the constraints of the Principle of Actualism. Obvious questions are “What about the time when there were no terrestrial processes?” and “Does Actualism cause a problem for astronomical applications?” Hutton’s formulation took a process-oriented approach which allows experimental verification (Rietmeijer & Nuth 2002). It paved the way to experimental verification of geological processes (experimental 1
The Astrophysical Journal, 771:34 (13pp), 2013 July 1
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Its related mineral roedderite was found among the dust of comet 81P/Wild 2 (Joswiak et al. 2007). Eifelite (KNa3 Mg4 Si12 O3 ) was formed by precipitation from a highly alkaline, Al-deficient, Mg–Si-rich supercritical hydrothermal solution inside vugs in a volcanic rock (Abraham et al. 1983). It is so far the only known terrestrial silicate mineral to do so. Although this does not establish a terrestrial process of vapor phase condensation of silicate minerals, it suggests that gas-to-solid vapor phase silicate condensation could be explored in dedicated simulation experiments. Of the three possibilities cited for forsterite and enstatite formation around Herbig Ae/Be stars, the only viable option, i.e., conforming to the Principle of Actualism, would be condensation and thermal annealing. By definition, equilibrium condensation (Grossman & Larimer 1974) yields stoichiometric crystalline solids, i.e., minerals, and thus cannot explain the presence of amorphous silicates with olivine- and pyroxene stoichiometry around Herbig Ae/Be stars (Bouwman et al. 2001; Molster & Waters 2003; Molster & Kemper 2005; Juh´asz et al. 2010). Only non-equilibrium vapor phase condensation produces amorphous solids, but with unpredictable compositions unless there is a pre-equilibrium regime that is characterized by cycles of condensation and evaporation (De 1979) that can lead to the formation of amorphous solids with predictable deep metastable eutectic (DME) compositions. From a geological point of view, dust formation around Herbig Ae/Be stars begins with the nonequilibrium vapor phase condensation of “amorphous silicates with olivine and pyroxene stoichiometry” followed by thermal annealing causing the formation of “crystalline forsterite, and enstatite and silica.” Both processes are amenable to dedicated laboratory vapor phase condensation and thermal annealing experiments, including shock processing. Juh´asz et al. (2010) observed: “It is an interesting and highly debated question how silicate crystals form in protoplanetary disks.” The purpose of this paper will be to apply experimental data to constrain dust-forming processes and evolution around Herbig Ae/Be stars from the Infrared Space Observatory (ISO) survey. We will discuss a selected set of Herbig Ae/Be disks from Bouwman et al. (2001) and incorporate results from the Spitzer Space Telescope survey (Juh´asz et al. 2010) as a framework to achieve this goal. We will (1) review laboratory experiments of Mg-silicate formation and annealing, (2) integrate the experimental data and the dust parameters developed by Bouwman et al. (2001), and (3) explore the petrologic implications of the dust-forming processes and minerals in Herbig Ae/Be disks.
Silicate evolution in Herbig Ae/Be disks is obviously determined by physical conditions in an evolving disk, e.g., the temperature structure of the disk, the role of transport processes, the role of shock waves, turbulent eddies, and transient heating events, including accretion outbursts in young stars. Effects caused by these processes cannot be assessed in vapor phase condensation experiments that follow the evolution of hot vapors into initially amorphous Mg-silicate dust and the formation of forsterite and enstatite that will gradually increase the crystalline to amorphous Mg-silicate ratio. We have previously suggested that relative comet ages can be obtained from the fraction of crystalline Mg-silicates, that is, “old” comets contain primitive amorphous silicate grains, whereas younger comets provide information on chemical processes and annealing in the solar nebula (Nuth et al. 2000a). Thermal annealing of initially amorphous grains in Herbig Ae/Be systems would increase the amount of crystalline silicates that could then be transported outward and become incorporated into comets (Nuth et al. 2000a). Still all experiments are simplifications of natural processes to some degree so that they can be carried out at reasonable temperatures and timescales. Experiments are not an exact replica of natural environments. Experiments may produce unnatural artifacts. For example, in addition to the abundant magnesiosilica nanograins, the condensation of Mg–SiO–H2 –O2 vapors also produced MgO and SiO2 nanograins (Rietmeijer et al. 2002b). It might be argued that both oxides had no time to coagulate into magnesiosilica nanograins inside the condensation chamber and are therefore artifacts. It might also be argued that such pure end-member compositions represent naturally stable condensation nuclei. To resolve this argument requires observations of the natural environment. It has long been recognized that the oldest materials in the solar nebula were the result of condensation and aggregation of dust (Smith 1979) that could then lead to mineral dust evolution leading to new minerals that could not be formed by condensation (Hazen et al. 2008). We submit that non-equilibrium vapor phase condensation of metastable amorphous Mg-silicates and coagulation of these dust grains could be the first stage of this process. These amorphous grains that formed well below their glass transition temperature are thermodynamically poised to equilibrate, but when they eventually reached equilibrium they will have done so by unpredictable pathways that were only constrained by the bulk chemical composition of the system. In this case, the end products are forsterite, enstatite, and silica that could form well below the equilibrium condensation temperature of these minerals during the 1–10 Myr lifetime of Herbig Ae/Be systems. Still, it is thought by some that crystalline astronomical silicates form by direct vapor phase condensation at temperatures between 1200 K and 1400 K, but these temperatures refer to forsterite and enstatite formation from a hot gas as predicted by thermodynamic equilibrium models (Hanner & Zolensky 2010), and such condensation behavior has never been observed in the laboratory in the absence of suitable substrates.
2. CONSTRAINTS AND LIMITATIONS Comparisons between Mg-silicates condensed from Mg– SiO–H2 –O2 vapors in the laboratory and Mg-silicates around Herbig Ae/Be stars are possible as it is reasonable to assume that Mg and Si in their relative cosmic abundances, Si:Mg = 1.0:1.07 (×106 ) (Anders & Grevesse 1989), as well as O and H, are present around these stars to form silicates. But it is no simple exercise of matching data sets. For example, the length and timescales of “astronomy-in-a-bell-jar” are measured in nano- and micrometers and over periods of seconds to days necessitating bold extrapolations from the laboratory experiments. For example, it is not practical to match the exact temperature–pressure conditions in Herbig Ae/Be disks or the initial gas phase chemistry such as Si–O speciation. This paper takes the process-oriented approach dictated by the Principle of Actualism.
3. TERMINOLOGY There seem to be different nomenclatures in use in Astronomy and the Earth Sciences. But since the former is the latecomer, we suggest that it follows the existing nomenclature for natural solids. As the word “silicate” already implies that the material is crystalline and could be a mineral, the terms “amorphous silicate” and “amorphous olivine” are mineralogical misnomers because the words “silicate” and “olivine” (likewise pyroxene) already refer to an ordered natural material, i.e., a mineral. 2
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Table 1 Mg-silicates, Magnesium Oxide, and Amorphous Magnesiosilica Compounds: Names, Formulae, and Abbreviations Used in this Paper (Rietmeijer 2009, 2011) Silicate Mineral
Mineral Name
Chemical Formula
Abbreviation
Olivine Pyroxene
Forsterite Enstatite
Mg2 SiO4 MgSiO3
Fo En
Oxide mineral Silica Silica Magnesium oxide
Tridymite Coesite Periclase
SiO2 SiO2 MgO
Tr ... ...
Serpentine-dehydroxylate Smectite-dehydroxylate High-MgO dehydroxylate
Amorphous magnesiosilica compounds ... Mg3 Si2 O7 ... Mg6 Si8 O22 ... Mg8 SiO10
... ... ...
Note. Mineral type, name, and chemical formulae are used interchangeably.
Crystalline silicate is generally redundant; there are no amorphous silicates. A mineral is defined as a naturally occurring solid with a highly ordered atomic arrangement and a definite (but not necessarily fixed) homogeneous chemical composition (Klein & Dutrow 2007). As all minerals are crystalline, identification of “crystalline forsterite, and enstatite and silica” are incorrect. Forsterite refers to a mineral with a stoichiometric Mg2 SiO4 composition. There is no amorphous forsterite! Likewise, enstatite refers to a mineral with an MgSiO3 composition; there is no amorphous enstatite. Silica (SiO2 ) has several polymorphs including tridymite and coesite, and both are minerals. Strictly speaking then an experimentally produced, well-ordered, that is, crystalline, solid with a stoichiometric Mg2 SiO4 composition, that is, forsterite, cannot be considered as a mineral, and likewise for enstatite and silica. This is a matter for purist mineralogists but as experiments are consistent with the process-oriented approach of the Principle of Actualism, we do not make a distinction between synthetic and natural minerals. The correct mineral designations are shown in Table 1. Finally, olivine and pyroxene form complete solid solutions between their pure-Mg and pure-Fe end members that in the case of olivine are Mg2 SiO4 and Fe2 SiO4 with an intermediate MgFeSiO4 composition and pyroxene ranging from MgSiO3 and FeSiO3 with an intermediate MgFeSi2 O6 composition. Historically, both solid solution series were subdivided by the Mg/(Mg+Fe) (mg) composition into distinctly named members. For example, olivine with mg = 60–80 would be hortonolite while a pyroxene mineral with mg = 10–30 would be called bronzite. A Ca-free Mg,Fe-pyroxene with mg = 100–80 is called enstatite (Deer et al. 1966). This practice linking olivine and pyroxene composition to differently named intermediaries has fallen into disuse. When the chemical composition of olivine and pyroxene crystals is known, it is indicated by the mg-value. The terms “crystalline olivine grains” and “crystalline pyroxene grains” would be correct though redundant when the exact composition is unknown. Nomenclature is more complex when dealing with amorphous solids, e.g., “amorphous silicates with olivine and pyroxene stoichiometry.” The word ‘silicates’ is incorrectly used when identifying an amorphous solid. The composition of an amorphous solid can be specified descriptively as, e.g., “an amorphous material of olivine composition” or when the exact composition is known as “an amorphous material of forsteritic composition.” Likewise, in this paper, it is appropriate to refer to “an amorphous material of pyroxene composition” or “an amorphous material of enstatitic composition”, and amorphous silica that
Table 2 Grain Size and Grain Agglomerate Sizes in Vapor Phase Condensed Amorphous Magnesiosilica Grains (Sources: (1a) Rietmeijer et al. 2002a; (2) Rietmeijer et al. 2002b; (3) Fabian et al. 2000), and Sizes of Coagulated Amorphous Condensates (Source: (1b) Rietmeijer et al. 2002a) Size (Diameter)
Ref.
Spherical grains
Grain agglomerates
1–39 nm; mean = ∼8 nm 13–123 nm; μ = 45 ± 17 nm 1–40 nm 10–100 nm 100 nm to almost 3 μm
10–195; μ = 117 ± 41 nm 100 nm that are necessary for the widespread nucleation and growth of forsterite, enstatite, and tridymite nanocrystals. The smokes for this thermal annealing experiment (Hallenbeck et al. 1998) were produced ab initio from gas phase species (Mg, SiO) that were premixed, passing through a flame front prior to entering the condensation chamber. In another ab initio vapor condensation experiment, the condensing SiO and Mg vapors were introduced separately from different crucibles at 875 K (Kaito et al. 2003). The resulting smokes formed (1) an open network of densely packed necklaces of amorphous SiO nanograins, (2) amorphous Mg spheres, and (3) ∼100 nm-sized hexagonal Mg plates. Mg-nanograins (80 to 130 nm) were polycrystalline. Condensed nanograins with a core of mixed Si metal and Mg2 Si crystals were covered by a 2 nm thick MgO layer (Kaito et al. 2003). In SiO-rich vapors, the condensed spheres of well-ordered forsterite (∼50 nm in diameter) had a layer of MgO micro-crystallites. Si metal crystallites had an amorphous metallic Si layer (Kaito et al. 2003). Forsterite nanocrystals in this experiment already showed considerable chemical and structural order. A stall phase occurred when nanocrystals were forming at grain surfaces. When surface coverage was complete and crystals penetrated into the grain, the IR spectral evolution began again, ending the stall phase. These experiments showed considerable lack of chemical equilibrium in the rimmed polymineralic condensate grains, which is reminiscent of the initial readjustments in the pre-stall phase in the experiments conducted by Hallenbeck et al. (1998, 2000). These experiments suggest that the initial nucleation and growth of “amorphous silicates with olivine and pyroxene stoichiometry” around Herbig Ae/Be could be a complex and largely unknown process that will affect the final outcome. Thermal annealing up to 1173 K (78 hr) of large glassy Mg-pyroxene grains up to ∼100 μm in size caused the rapid formation of forsterite (Thompson & Tang 2001). No further changes occurred; only “sharpening of the crystalline feature” in X-ray powder diffraction patterns that is consistent with
magnesiosilica grains with olivine stoichiometry that rapidly crystallized as metastable forsterite nanocrystals. Other amorphous magnesiosilica smokes were produced by laser ablation of “glassy silicate” targets with a pyroxene or olivine composition. The smallest grains in these experiments (Table 2) had “non-stoichiometric silicate compositions” (Fabian et al. 2000). They resembled amorphous magnesiosilica grains with a smectite-dehydroxylate composition. The amorphous magnesiosilica grains showed significant grain coagulation (see Figure 1; Fabian et al. 2000). Large amorphous grains ranging from 1 to 2 μm had forsteritic and enstatitic compositions and contained scattered forsterite nanograins. 4.2. Thermal Annealing of Amorphous Mg-silicates: A Stall Phase The initial IR-spectral changes induced in metastable amorphous magnesiosilica produced in the CFA during controlled thermal annealing were readjustments of the Si–O stretching and O–Si–O bending vibrations indicating oxidation/reduction reactions to produce fully oxidized amorphous magnesiosilica with entrapped metal atoms (Hallenbeck et al. 1998, 2000). Continued annealing caused no further IR-detectable changes for some (temperature-dependent) time period. The amorphous magnesiosilica evolution had stalled with no further mineralogical changes. This marked the onset of grain growth by coagulation of condensed amorphous magnesiosilica nanograins (Rietmeijer et al. 2002a). Changes in grain size occurred primarily in amorphous magnesiosilica grains with a serpentinedehydroxylate composition. These grains fused into round amorphous magnesiosilica grains up to ∼75 nm in diameter (Figure 1). By contrast, noticeable grain growth in more porous smokes first occurred only well into the post-stall phase, and then only for the largest grains (Figure 1). A few forsterite and tridymite grains 100 nm size grains that were produced in ab initio vapor phase condensation based on the post-stall data in a thermal annealing experiment (solid squares; from Hallenbeck et al. 1998, 2000). The dashed line is an extrapolation to lower temperatures. The dotted line boldly extrapolates experimental data for condensed samples that required no induction or stall phase to nucleate Fo + Tr crystallites in amorphous magnesiosilica materials (dots: Davoisne et al. 2006; open squares: Fabian et al. 2000; open triangles: Rietmeijer et al. 1986; open diamonds: Thompson & Tang 2001). The horizontal lines delineate the 1–10 Ma ages of Herbig Ae/Be stars (modified after Helling & Rietmeijer 2009).
10 Myr at ∼600 K. These lower temperatures were derived after an extreme linear extrapolation of experimental data obtained at much higher temperatures. To the best of our knowledge, there are no experimental data on thermal annealing of amorphous magnesiosilica to fill the gap between 600 K and 900 K. Such experimental studies are likely to require considerably longer timescales than a single human lifespan. 6.2. Crystalline Silicate-forming Reactions Large (>100 nm) DME amorphous magnesiosilica grains offer only limited pathways for the crystalline evolution of forsterite, enstatite, and tridymite from amorphous serpentinedehydroxylate (#1; #2), smectite-dehydroxylate (#3; #4), or high-MgO dehydroxylate (#5, #6) grains (Table 3). These primary reactions show how these amorphous grain compositions (1) define the forsterite/tridymite ratio (#1, #3), (2) co-produce Fo and En in different relative fractions (#2, #4), (3) yield Fo + En + Tr (#4), and (4) yield MgO (#5, #6). The observation that SiO2 abundances around Herbig Ae/Be stars tend to increase with increasing degrees of Mg-silicate crystallinity (Bouwman et al. 2001) is consistent with these primary reactions. A high 8
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Figure 6. Mforst /(Mforst + MSiO2 ) ratios vs. m2.0 /m0.1 ratios (Table 5) for selected group Ia and IIa Herbig Ae/Be systems. Mforst stands for forsterite but includes enstatite when present. The vertical lines represent primary reactions for silicate formation in amorphous magnesiosilica grains with a serpentine-dehydroxylate composition (reaction #1) and a smectite-dehydroxylate composition (reactions #3 and #4; Table 3). An Mforst /(Mforst + MSiO2 ) = 1 defines the high-MgO dehydroxylate magnesiosilica grains (#5).
pure forsterite to low-Fe, (Mg,Fe)-olivine (Koike & Tsuchiyama 1992). These examples highlight that Mg-silicates condensing from vapors produced by evaporating natural olivine and pyroxene may inherit the original mineral composition. Thus, transient ´ high-temperature events around Herbig Ae/Be stars (Abrah´ am et al. 2009) could also cause re-condensation of amorphous magnesiosilica grains of olivine and pyroxene composition that might then be annealed to crystallinity.
Table 5 Calculated Mass Fractions of Mg-silicates Forsterite (± enstatite) and Tridymite, Mforst /(Mforst + MSiO2 ) Using the Mforst /Msil and MSiO2 /Msil Ratios (Original Data from Bouwman et al. 2001) and the m2.0 /m0.1 from the Same Source
HD 179218 HD 142527 HD 100546 HD 163296 HD 104237 HD 144432 HD 142666 HD 150193 Halley
6.3. Periclase (MgO) The presence of MgO in Herbig Ae/Be systems was inconclusive (Molster & Waters 2003); it was absent in the Spitzer survey (Juh´asz et al. 2010). The premise of the primary reactions (Table 3) is that amorphous magnesiosilica grains formed by non-equilibrium vapor phase condensation. Primary reactions #5 and #6 yield a significant amount of MgO but amorphous grains with the high-MgO dehydroxylate composition are extremely rare (Rietmeijer et al. 2004). They may be ignored for all practical purposes. Periclase is also a reaction product of secondary reaction #7. This reaction either did not take place in Herbig Ae/Be disks or the amount of MgO is too small to be detectable, but its spectral IR profile is known (Hofmeister et al. 2003).
m2.0 /m0.1
Mforst /Msil
MSiO2 /Msil
Mforst /(Mforst + MSiO2 )
2.86 16 >57 0.42 8 0.86 1.54 0.29 2.73
0.092 0.058 0.140 0.038 0.072 0.075 0.013 0.045 0.220
0.022 0.026
